This invention relates to the reduction of intermodulation distortion in broadband linear RF power amplifiers. More specifically, it relates to the cancellation of the intermodulation distortion through the application of a DSP based negative feedback derived error signal at the input of the amplifier.
There have been several different techniques developed for the linearization of nonlinear RF circuitry, in particular the reduction of in-band intermodulation distortion produced by high frequency linear power amplifiers. These methods may be characterized as one of feed forward, feedback or pre-distortion correction. For low frequency applications, negative feedback has clearly been the linearization technique of choice, but in high frequency circuits the application of negative feedback has been limited.
The use of traditional analog negative feedback has several performance penalties. The distortion products are reduced by an amount equivalent to the loop gain, but so is the gain of the amplifier. At lower frequencies high gain devices are common, but at RF frequencies the gain of the available active devices is low, so as a result more stages may be required for a given gain requirement. Another issue is the amount of time delay inherent in the feedback loop. This delay, which may be sufficient for several cycles of the RF signal, results in a frequency dependent phase shift within the feedback loop. This additional phase has two effects on the performance of the amplifier: first it reduces the bandwidth over which the negative feedback is effective, and secondly it tends to make the amplifier more unstable and thereby reduces the amount of loop gain that can be used. Several techniques have been disclosed in the prior art to improve the performance of the negative feedback amplifier.
N. Sokal et al., U.S. Pat. No. 3,900,823 on xe2x80x9cAmplifying and Processing Apparatus for Modulated Carrier Signalsxe2x80x9d, issued Aug. 19, 1975, propose a method for linear amplification of single side-band modulated signals. A negative feedback signal is developed by taking the difference between the amplifier""s modulated input and output signals, and then using that error signal to adjust an amplifier""s gain by controlling the input power supply voltage. This circuit is successful for narrow band modulation only.
M. Y. Huang, U.S. Pat. No. 4,276,514 on xe2x80x9cWideband, Phase Compensated Amplifier with Negative Feedback of Distortion Components in the Output Signalxe2x80x9d, issued Jun. 30, 1981, discloses a negative feedback technique for RF power amplifiers by which the input signal is removed from the feedback loop such that only the distortion components remain in the loop. As a result, the gain of the amplifier is unaffected while the distortion products are reduced by an amount equivalent to the loop gain. Huang attempts to address the effect of the time delay in the feedback loop by adding delay equalization into the loop, however the circuit remains narrow band in nature. A second performance penalty is in the addition of a power divider at the circuit input in order to feed the input signal into the cancellation circuit for the loop. The additional loss caused by the divider reduces the gain of the overall circuit, although not as much as would be expected from the loop gain. This particular circuit has been independently disclosed several times in the literature, by McRory and Johnston (1993), Faulkner and Briffa (1995), and Villanueva and Aguilar (1996).
The cancellation of the input signal from the feedback loop was also implemented by L. V. Hayes (U.S. Pat. No. 5,396,189, xe2x80x9cAdaptive Feedback Systemxe2x80x9d, issued Mar. 7. 1995), who made the system adaptive, and by B. J. Budnik (U.S. Pat. No. 5,745,006, xe2x80x9cMethod of Compensating for Distortion in an Amplifierxe2x80x9d, issued Apr. 28, 1998), who included adaptive pre-distortion.
The effect of the loop delay has also been addressed. K. G. Voyce et al., U.S. Pat. No. 4,929,906, xe2x80x9cAmplifier Linearization Using Down/Up Conversionxe2x80x9d, issued May 29, 1990, teaches of a method that performs the negative feedback at IF frequencies. In this case, both the input signal and the amplifier""s output signal are down converted to an IF frequency. The effect of the loop delay at the IF frequency is minimized as the resulting phase shift is much smaller at the lower frequency, potentially increasing the amplifier""s stability and hence increasing the available loop gain. However, the addition of up and down conversion in the loop greatly increases the complexity of the circuit and will introduce conversion loss into the loop, requiring a larger PA for equivalent output power.
J. F. Cleveland, U.S. Pat. No. 5,237,288, xe2x80x9cRF Power Amplifier Linearizationxe2x80x9d, issued Aug. 17, 1993, discloses a method of optimizing the negative feedback by correcting the phase transfer function of the feedback loop such that it operates at the required xcfx80 radians phase shift for each operating frequency. Although this method optimizes the negative feedback condition, it cannot improve the amplifier""s stability and hence the maximum loop gain available in this case is 6 dB.
J. R. Melton, U.S. Pat. No. 5,783,968, xe2x80x9cRF Amplifier Method and Apparatusxe2x80x9d, issued Jul. 21, 1998, discloses a method in which the down/up conversion technique and the tuning of the feedback loops phase response is combined. In this case the feedback and error signal derivation is performed at RF frequencies, but the error signal is then down converted at processed at baseband. The amplifier is tuned during a stabilization phase during which the phase transfer function of the loop is set to the optimum condition.
When considering the prior art, it is clear that the use of negative feedback in the linearization of high frequency circuits is limited in its effective bandwidth and in the amount of loop gain available due to stability consideration. Furthermore, the prior art shows that in order to improve the performance of the negative feedback, additional circuitry is required that contributes its own performance penalty in loss, additional distortion, and reduced system efficiency.
A principal object of this invention is to provide a DSP based negative feedback derived linearization technique that will reduce intermodulation distortion, will not require additional RF hardware, will be capable of linearizing an entire transmit chain, is easily trainable for adaptive applications, and is broadband in capability.
Although analog circuitry can be used to improve the performance of negative feedback, the application of DSP technology offers a better solution for the linearization of a broadband amplifier. The basic idea of the invention is to use a complex baseband model of the power amplifier within a DSP domain to develop a feedback signal that would be equivalent to the optimum negative feedback used for the analog amplifier. Once the feedback signal is available, it can be processed to compensate for the effects of the group delay and for optimum loop gain, hence resulting in a broadband response with no theoretical limitations on the linearization of the amplifier.
Therefore, according to an aspect of the invention, there is provided apparatus for achieving RF amplification while reducing intermodulation distortion, the apparatus comprising an amplifier having an input and an output; and a processor incorporating a model of the amplifier, the processor having an output connected to the input of the amplifier such that the output of the processor comprises a desired input signal of the amplifier combined with a broadband feedback error signal to reduce the intermodulation distortion of the amplifier.
Preferably, the processor comprises a complex baseband model of the amplifier that replicates the output complex envelope of the amplifier in the absence of combination of the input of the amplifier with the broadband feedback error signal.
In a further aspect of the invention, there may be provided a first multiplier to apply a gain of 1+xcex1 to the input signal which is then combined with the error signal and fed to the amplifier. In a further aspect of the invention, there may be provided a second multiplier to apply a gain of xcex1 to the input signal which is fed to the complex baseband model; and a third multiplier to apply a gain of xcex2 to the output of the complex baseband model such that the scaled output of the model is equivalent to a broadband feedback error signal which is then combined with the scaled input signal to form the output of the processor.
In a further aspect of the invention, there may be provided, on the input to the amplifier after the output of the processor, a digital to analog converter followed by an anti-aliasing filter; and an up converter followed by a bandpass filter. The amplifier may be a feed-forward amplifier, and the amplifier and the model may be contained in an up-converter and transmit chain. Preferably, the model is adaptive to the thermal condition of the amplifier. The output of the amplifier may also applied to the model for training the model.